1. Field of the Invention
This invention is concerned with security paper for banknotes, cheques and like documents and also with other security articles including credit cards or like plastic articles which are required to provide a high degree of security against imitations.
2. The Prior Art
It is widely known to use in banknotes and other security documents, security devices such as security elements, e.g. security threads or security strips, which are made from a transparent film provided with a continuous reflective metal layer, such as aluminium, which is vacuum deposited on, for example, polyester film. Banknotes made from such paper have been in general circulation in many countries for many years. When security devices are embedded in the security paper and the paper is subsequently printed to provide the security documents, the thread cannot be readily discerned in reflective light but is immediately transparent as a dark image when the document is viewed in transmitted light.
Further, in our previous British patent specification No 1,095,286 there is described and claimed a security device for use in security paper comprising a continuous fine security ribbon having a width of substantially 0.75 mm and having printed thereon a design, lettering or pattern comprising printed characters of a height of substantially 0.4 mm. The security ribbon described in GB 1,095,286 may be made of metal foil which may be aluminium and furthermore may be in the form of a laminate; the printed design, lettering or pattern as disclosed in the specification is very small, i.e. 0.4 mm, and is not readily visible without an aid to vision, such as a magnifying glass.
In recent times, in order to enhance the security of security documents, especially banknotes, against modern counterfeiting techniques making use of sophisticated colour separation, printing and colour photocopier technology, it has become common to use a security thread comprising a thin layer of aluminium on a plastic support which is exposed on one side of the sheet at intervals along the length of the thread, the region of exposure being referred to as a window. British Patent Specifications Nos. 1,552,853 and 1,604,463 disclosed banknotes containing such windows. Paper for use in producing such banknotes can be made using the method disclosed in our European Patent Specification No. 0 059 056. The widespread use of banknotes having security thread exposed in windows along the length of the thread has resulted in enhanced security. A banknote of this type provides added security against counterfeiters as, when viewed in transmitted light, the strip is seen as a dark line, and, when viewed in reflective light on the appropriate side, the bright shiny aluminium portions which are exposed at the windows are readily visible. However, there is a need for even greater security for banknotes and like documents whether or not the security device is exposed in windows.
The present invention therefore is concerned with providing a novel security element, which may be in the form of a strip or thread, of enhanced security to provide security paper for banknotes, cheques and the like which is even more difficult to counterfeit than present banknotes. Also the invention is concerned with security articles including credit cards, identity badges and travel tickets which comprise the novel security element of this invention.